1. Field of the Invention
The present invention generally relates to the field of analog phase shifters and more specifically, to analog phase shifters using voltage variable capacitors.
2. Description of the Related Art
All-pass networks are used in communication electronics for phase-compensation and phase-shifting networks. The transfer H(ω) for a simple fourth-order all-pass network has the form
                              H          ⁡                      (            ω            )                          =                                            ω              2                        +                          a              ⁢                                                          ⁢              j              ⁢                                                          ⁢              ω                        -                          b              2                                                          ω              2                        -                          a              ⁢                                                          ⁢              j              ⁢                                                          ⁢              ω                        -                          b              2                                                          (        1        )            where ω is frequency, and a and b are constants.
Consequently, the amplitude response is a constant and only the signal phase is affected. Fourth-order lumped-element all-pass networks can be constructed from “bridged-tee” circuits. FIGS. 1a and 1b illustrate conventional bridged-tee circuits and design equations for realizing all-pass transfer functions. Specifically, FIG. 1a illustrates a conventional bridged low-pass tee and FIG. 1b illustrates a conventional bridged high pass tee. The symbols labeled C1 and C2 are capacitive elements with capacitance of C1 and C2 respectively. Similarly, the symbols labeled L1 and L2 are inductive elements with inductance of L1 and L2 respectively.
The relevant design equations are also given with these figures, where R is the desired input/output impedance of the circuit, ω0 is the center frequency for the bridged-tee circuit, and the capacitances C1 and C2 and inductances L1 and L2 are defined as shown in the FIGS. Note that
                              C          2                =                  4          ⁢                      C            1                                              (        2        )                        and                                                      R        =                                            2              ⁢                              L                1                                                    C              2                                                                      where C1, C2 and L1 are the capacitances and inductance shown in FIG. 1a. 
Two related conventional all-pass sections are shown in FIGS. 2a and 2b. These sections include a shunt resonator element to achieve a more rapid variation in phase near the center frequency, specified by the Q-factor in the design equations. As with FIGS. 1a and 1b. the symbols labeled C1 and C2 are capacitive elements with capacitance of C1 and C2 respectively, and the symbols labeled L1 and L2 are inductive elements with inductance of L1 and L2 respectively. In the design equations shown, the terms have the same definitions as in FIGS. 1a and 1b. and the new term Q is the Q-factor. In FIGS. 2a and 2b, the conventional all-pass networks are illustrated with Q≧1. These circuits reduce to that of FIGS. 1a and 1b when Q=1.
The phase response of a conventional phase shifter circuit using bridged-tee networks can be varied electronically by making the capacitive elements voltage-tunable, as indicated by the arrow through the capacitor symbols. The symbols labeled C1 and C2 are voltage-tunable capacitive elements with capacitance of C1 and C2respectively, and the symbols labeled L1 and L2 are inductive elements with inductance of L1 and L2 respectively. FIG. 3a illustrates a conventional bridged low-pass tee with a voltage tunable capacitive element and FIG. 3b illustrates a conventional bridged high pass tee with a voltage tunable capacitive element.
FIG. 4 illustrates a response of a single, ideal all-pass section with R=1 and ω0=1 and Q=1 as the capacitors are varied by a factor of two around the nominal design values specified in FIGS. 1a and 1b. Note that the maximum phase-shift occurs at the “center-frequency” of the design. Thus by properly choosing the inductor and capacitor values, the circuit can be designed for a prescribed center-frequency and characteristic impedance. Usually (but not always) the characteristic impedance is set at 50 Ohms for an RF circuit.
Phase-shifters based on lumped-element all-pass sections such as those in FIGS. 2a and 2b have been demonstrated with several different technologies for tunable capacitors. The article “A precise analog phase-shifter for SHF SATCOM phased arrays (N. E. Hodges and M. H. Yam, 1992 IEEE GaAs IC Symposium Digest, pp. 29-32) describes a phase-shifter using diode varactors. The article “A MMIC active phase shifter using variable resonant circuit” (A. Hayashi and M. Muraguchi, IEEE Transactions Microwave Theory Tech., vol. 47, Oct. 1999, pp. 202 1-2026) and the article “A tunable all-pass MIC active phase shifter (D. Viveiros, D. Consonni, and A. K. Jastryebski, IEEE Transactions Microwave Theory Tech., vol. 50, August 2002, pp. 1885-1889) describes an all-pass phase-shifter using GaAs MESFETs. The article “2.4 GHz Continuously Variable Ferroelectric Phase Shifters Using All-Pass Networks” (D. Kim et al., IEEE Microwave and Wireless Components Left, vol. 13, Oct. 2003, pp. 434-436) describes an all-pass phase-shifter using capacitors made from tunable dielectric materials. However, electronic phase-shifters using all-pass section are not common, and at least one prominent textbook in this field fails to mention the topology (see, S. Koul and B. Bhat, Microwave and Millimeter-wave Phase Shifters, vol. II: Semiconductor and Delay-Line Phase Shifters, Artech House: Boston, Mass., 1991.
One reason for the lack of interest in all-pass phase-shifter is that in many applications such as phased-array antennas, a voltage-variable phase-shift of up to 360 degrees is desirable. This is not practical with a single all-pass section, because extremely large capacitance tuning ratios are required, and the resulting changes in the characteristic impedance of the circuit result in severe impedance mismatches with the host system and consequently poor insertion-loss characteristics. In particular, a conventional single-all pass section provides a phase-shift of barely up to 90 degrees.
One attempt at solving this problem has been described in the Hodges and Yam article and the Kim et al. article previously referenced, where two identical all-pass sections have been cascaded to increase the phase-shift. However, this approach suffers from several drawbacks. First, the phase-versus-frequency response is not constant, as is desired in many applications. Second, the variation in characteristic impedance as the capacitors are tuned has an increasingly pronounced effect as the number of sections increases. This is severely exacerbated by non-idealities in the lumped elements, such as interwinding capacitance in the inductive elements, mutual inductive coupling between the inductive elements, or parasitic inductance in the interconnecting lines between adjacent all-pass sections.
From the above, there is a need for a phase shifter that provides phase shift over almost 360 degrees (or greater) and is relatively constant over a wider frequency range with a limited amplitude modulation over the range.